Drug users caught with as few as five ecstasy tablets or five grams of cannabis - enough for about 10 joints - will be prosecuted as dealers under regulations drawn up by the Home Office, the Guardian has learned. The plan to slash the limit for cannabis possession for personal use would mean that anyone found with more could face a prison sentence of up to 14 years.

The Home Office has written to the government's experts, the Advisory Committee on the Misuse of Drugs, telling them that ministers are also minded to set the threshold for possession for heroin and cocaine at two grams.

Although home secretary John Reid has yet to take a final decision, draft regulations seen by the Guardian - to be introduced into parliament shortly under last year's drugs act - will mean that those found with more than these specified amounts would be charged with possession with intent to supply. Under the act, dealers of cocaine and heroin face a maximum of life imprisonment. The plan for a 5g cannabis threshold marks a sharp reversal from David Blunkett's decision 18 months ago to ensure that cannabis possession was normally to be dealt with by confiscation and an informal warning.

The proposed thresholds are so low that the advisory committee, which discussed the issue on May 25, is believed to have warned the Home Office that they would cause policing problems. The committee suggested the cannabis threshold should be set at 28g, or 1oz. The experts also told ministers that the five tablet limit for ecstasy was low - given that they can be bought for 50p each in some areas, and some users take up to 10 in one session.

The Home Office letter to the ACMD, seen by the Guardian, says that ministers are setting thresholds at this stage only for the drugs which cause the most harm or which are most prevalent - heroin, crack, cocaine, ecstasy, amphetamines, and cannabis. It adds that the proposed levels for all the drugs - except amphetamines - are considerably lower than those originally proposed, because most respondents to a consultationon the proposals considered the limits excessive.

The government now proposes the following thresholds:

Cannabis

Ministers propose 5g, or less than 1/5th of ounce - enough for 10-20 joints. This compares with the original proposal of 4ozs or 133g of resin, and 500g or 20 bags of grass. The ACMD has replied that the limit should be set at 28g.

Ecstasy

Ministers propose 1.5g (equal to 5 tablets, costing £15), compared with an original proposal for 10 tablets. The Home Office says it would be more straightforward to do it by weight than number of tablets, as the drug also comes in powder form. The ACMD said the limit should be 2g or 20 tablets, as that was two days' supply.

Amphetamines

Ministers have kept the proposed threshold at 14g but dropped an alternative of 10 x 1g wraps, saying dealers would simply change the size of deals to avoid going above the threshold. The ACMD said the threshold should be 10g, and questioned the rationale for a threshold higher than other drugs.

Heroin, cocaine and crack cocaine

Ministers are "minded to set" a threshold of 2g for possession, compared with the original proposal of 7g. The proposed number of individual wraps - a maximum of 10 in each case - has also been dropped for these class A drugs.

When the ACMD's technical committee considered the issue in April, it was pointed out that even Sir Ian Blair, the Metropolitan police commissioner, had misunderstood the proposals: "Many people still think that the provisions are about setting levels that are reasonable for personal us,e and that if they are caught with amounts below the thresholds they will not be arrested for possession with intent to supply. The reality is contrary to this." Martin Barnes, chief executive of Drugscope, an information charity, said this confusion had made ministers far more cautious. "We are concerned at the amounts being considered. The rationale for some thresholds remains unclear, and it is uncertain how many more people may be prosecuted with the more serious charge of intent to supply."

Paul Flynn, a Labour MP and drugs campaigner, said he hoped the ACMD would "give the proposals the attention they deserve, given that they come from a department in chaos. Let's hope they throw them out. I am sure that many people will throw up their hands in horror at this."

In January, Mr Reid's predecessor at the Home Office, Charles Clarke, confirmed Mr Blunkett's decision to downgrade cannabis from class B to class C. Mr Clarke conceded that the move had created confusion over the drug's legal status, but said it was based in part on the fact that the reclassification had not led to an increase in use among young adults, contrary to his expectations.

At the same time, Mr Clarke announced a crackdown on British cannabis farms and a public education campaign to stress both the harm and the illegal status of the drug.